Support build-depends: injection from cabal.project

[lawcho]

Proposal

Support the build-depends: field in a cabal.project file.

For example, this cabal.project file:

package *
  build-depends: ghc-tags-plugin
  ghc-options:
    -plugin-package=ghc-tags-plugin
    -fplugin=Plugin.GhcTags

would tell cabal build to build every component as if ghc-tags-plugin had been added to its build-depends:.

Motivation

I'm looking for a user-friendly way to run global GHC plugins ¹ with Cabal.

Currently, users of ghc-tags-plugin are asked to:

1. cabal install --lib ghc-tags-plugin
2. Find out where their package-db is
   (for me it was /home/lawrence/.local/state/cabal/store/ghc-9.4.8/package.db/)
3. Write a cabal.project similar to:

   package *
     ghc-options:
       -package-db=/home/lawrence/.local/state/cabal/store/ghc-9.4.8/package.db/
       -plugin-package=ghc-tags-plugin
       -fplugin=Plugin.GhcTags

4. cabal build

Related Issues

If #7901 is implemented, cabal.project would only need to contain:

package *
  plugin-depends: ghc-tags-plugin

And no cabal install --lib call would be required.

Unfortunately, a full implementation of #7901 'will be messy': #7901 (comment).

This feature might be easier to implement that #7901, because:

• Cabal already supports build-depends: in .cabal files
• No GHC-specific details need to be considered (build-depends: is compiler-agnositc)
• No plugin-specific details need to be considered (build-depends: works with any library)

Footnotes

1. A GHC plugin is global if it needs to run on all transitive dependencies of a package. (e.g. cabal-audit, ghc-tags-plugin, ghc-wpc) ↩

[gbaz]

I commented on the other issue, as the initial version of that is the same as this, except with plugin-depends instead of build-depends used in cabal project files. I prefer that, because I think build-depends should be what a package knows it depends on, and not be injectable.